This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-065235, filed Mar. 11, 1999, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a cellulose acetate structure excellent in its biodegradability and to a tobacco filter.
2. Description of the Related Art
Cellulose acetate is widely used for various structures, e.g., filter raw material, fiber for preparation of a woven fabric for clothing, a film, and molded articles obtained by, for example, injection molding or extrusion molding. A typical example of the cellulose acetate structure is a fiber. Particularly, almost all the tobacco filter is formed of cellulose acetate fiber.
Cellulose acetate fiber is manufactured nowadays as follows. First, flakes of raw material cellulose acetate are dissolved in a solvent such as acetone so as to prepare a spinning stock solution of cellulose acetate. The spinning stock solution thus prepared is spun into a fiber by a dry spinning method in which the spun fiber is discharged into a high temperature atmosphere, thereby obtaining the cellulose acetate fiber. It is also possible to employ a wet spinning method in place of the dry spinning method.
Particularly, in order to facilitate the manufacture of the tobacco filter, the cellulose acetate fiber used as the tobacco filter raw material is set at an appropriate total degree of fineness so as to be finished as a fiber tow. The tobacco filter is manufactured by fibrillating the cellulose acetate fiber tow by a filter plug making apparatus, followed by adding a plasticizer to the fibrillated cellulose acetate fiber and forming the resultant mixture into a rod by using a filter wrapper paper sheet and subsequently cutting the rod into pieces each having a predetermined length.
Cellulose acetate is an acetic acid ester of cellulose and is, essentially, biodegradable. In practice, however, the biodegradability of cellulose acetate is not necessarily high.
For example, the tobacco filter made of cellulose acetate fibers retains its shape even if the tobacco filter is kept buried in the soil for one or two years. A very long time is required for the tobacco filter to be completely biodegraded.
The tobacco filter is incorporated in the tobacco article so as to be circulated to the consumer and, after smoking, is discarded as the tobacco butte. Also, the tobacco filter is discarded directly from the filter manufacturing factory as the residue of manufacture. These discarded tobacco filters are treated as rubbish and in some cases are buried in the ground for disposal. Also, it is possible that the tobacco butte is not collected as rubbish, and is left to stand under the natural environment. These situations apply to not only the tobacco filter but also the general cellulose acetate structure.
Under the circumstances, vigorous research is being conducted on the biodegradation of cellulose acetate. It is reported as a result of such research that the biodegradation rate of cellulose acetate is dependent on the DS (Degree of Substitution: the number of acetyl groups per glucose unit skeleton) of cellulose acetate. To be more specific, if the DS of cellulose acetate is decreased, the biodegradation rate of cellulose acetate is promoted. The biodegradation mechanism of cellulose acetate is considered to be as follows.
In the first step, the acetyl group of cellulose acetate is cut by the exoenzyme released from microorganisms, with the result that the DS of cellulose acetate is decreased. Then, the cellulose acetate with the decreased DS is easily subjected to enzyme decomposition by, for example, cellulase, widely present in the environment and is finally subjected to the microorganism metabolism so as to be decomposed into carbon dioxide and water. The rate-limiting step of the biodegradation rate is considered to reside in the first cutting of the acetyl group. Also, the biodegradation rate of the structure of not only the cellulose acetate but also the overall plastic material is said to be also dependent on the surface area of the structure. To be more specific, if the material is the same, the biodegradation rate is increased with an increase in the contact area per unit weight with the microorganism environment. In other words, to increase the surface area of the plastic structure is to enhance the chance of contact with the decomposing bacteria.
Some methods of promoting the biodegradation rate of cellulose acetate are being proposed on the basis of the ideas described above. For example, it is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-199901 that an acid compound having an acid dissociation constant larger than that of acetic acid is added to cellulose acetate. However, if this method is employed in the present manufacturing process of the cellulose acetate fiber, as soon as an acid compound is added to cellulose acetate, the acetyl group of cellulose acetate is subjected to the chemical hydrolytic reaction under the influence of the acid compound. The chemical hydrolytic reaction of the acetyl group of cellulose acetate, which is carried out in the presence of the acid compound, causes DS of the cellulose acetate to be lowered so as to generate acetic acid. In other words, the acetyl group released from cellulose acetate is liberated as acetic acid. It follows that, in this prior art, the cellulose acetate structure is caused to generate a strong acetic acid odor under the influence of the liberated acetic acid. The acetic acid odor is not a desirable factor in the article. In the case of, for example, a tobacco filter, the generation of the acetic acid odor markedly impairs the taste of the tobacco.
It is also disclosed in PCT National Publication No. 7-500385 that a water-soluble compound and an organic compound capable of decomposition by bacteria are added to cellulose acetate. In this method, the added water-soluble compound and the organic compound capable of decomposition by bacteria elution into water under the natural environment and, then, are decomposed by bacteria so as to be released from cellulose acetate. As a result, the cellulose acetate structure is broken so as to increase the surface area of the structure. Since the surface area is increased, the biodegradation rate of cellulose acetate is increased. However, this prior art is intended to increase the chance of contact between cellulose acetate and the cellulose acetate decomposing bacteria, and does not essentially strengthen the functions of cellulose acetate and the cellulose acetate decomposing bacteria. Therefore, this prior art fails to produce the sufficient effect of promoting the biodegradation rate. Also, it is very difficult to mix these compounds in the cellulose acetate fiber in the present manufacturing process of the cellulose acetate fiber.
Further, proposed are several methods for adding microorganisms capable of biodegrading cellulose acetate or various decomposing enzymes of these microorganisms to cellulose acetate fiber. For example, proposed in Jpn. Pat. Appln. KOKAI Publication No. 8-70852 is a method of allowing a microorganism capable of deacetylating cellulose acetate and the deacetylating enzyme produced by the enzyme to be supported by cellulose acetate. These methods are considered to be effective for promoting the biodegradation rate of cellulose acetate. However, each of the additives used in these methods is costly, leading to a considerable increase in the cost of the cellulose acetate structure. Also, in view of mass production of tobacco filters, it is very difficult to use these additives that are not adapted for mass production. Further, it is very difficult to add the additives used in these methods to the cellulose acetate fiber in the present manufacturing process of cellulose acetate fiber. In using these additives, it is unavoidable to change markedly the manufacturing process of the tobacco filter made presently of the cellulose acetate fiber, making it very difficult to put these additives to practical use.
Also proposed are several methods in which a tobacco filter having an excellent biodegradability can be obtained by combining cellulose acetate low in biodegradability with another material high in biodegradability. For example, Jpn. Pat. Appln. KOKAI Publication No. 8-140654 discloses the method of coating the surface of the cellulose fiber such as wood pulp with a cellulose ester such as cellulose acetate. In these methods, however, the biodegradation rate of the entire tobacco filter is determined by the biodegradation rate of the material that can be easily biodegraded and, thus, the biodegradation rate of the cellulose acetate itself is not essentially increased.
Under the circumstances, the present invention is basically intended to provide a means for essentially promoting the mutual function between cellulose acetate and the cellulose acetate decomposing bacteria so as to further improve the biodegradability of cellulose acetate under the natural environment.
To be more specific, the present invention is intended to provide a cellulose acetate structure excellent in its biodegradability while suppressing the deterioration of the quality, particularly, suppressing the generation of the acetic acid odor, and a tobacco filter using the particular cellulose acetate structure.
The present invention is also intended to provide a biodegradable cellulose acetate structure that can be manufactured without markedly changing the existing manufacturing method.
As a result of an extensive research conducted in an attempt to overcome the above-noted problems inherent in the prior art, the present inventors have found that the biodegradability of cellulose acetate can be markedly improved by adding a predetermined compound to cellulose acetate, particularly by adding the predetermined compound to a spinning stock solution and spinning a fiber by using the spinning stock solution when the cellulose acetate is prepared in the form of a fiber.
According to the present invention, there is provided a cellulose acetate structure, at least a surface region of which comprises a biodegradable cellulose acetate composition comprising a biodegradation promoting agent contained in cellulose acetate and comprising at least one compound selected from the group consisting of a salt of an oxygen acid of phosphorus, an ester of an oxygen acid of phosphorus or a salt thereof, carbonic acid and a salt thereof.
It is desirable for the cellulose acetate to have a DS value falling within a range of between 2.0 and 2.6.
The biodegradation promoting agent should desirably have a solubility in water of room temperature of 2 g/dm3 or less. Such a biodegradation promoting agent can be selected from the group consisting of cellulose phosphate, starch phosphate, calcium secondary phosphate, calcium tertiary phosphate, and calcium phosphate hydroxide.
Where the biodegradation promoting agent is in the form of fine particles, it is desirable for the biodegradable cellulose acetate composition to further contain a dispersant for dispersing the fine particles in the composition.
In the present invention, it is desirable for the biodegradable cellulose acetate composition to further contain a photodegradation promoting agent.
The cellulose acetate structure of the present invention may be in the form of fibers or in the form of an unwoven fabric formed of short fibers having a length of 1 to 100 mm.
Further, according to the present invention, there is provided a tobacco filter comprising the cellulose acetate structure of the present invention in the form of fibers or a unwoven fabric.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.